The present invention relates to plate-like composite ferrite particles which have an appropriate coercive force, a large magnetization and a narrow coercive force distribution, plate-like composite ferrite particles which further has a large anisotropy field, and a process for producing such plate-like composite ferrite particles.
As described, for example, in Japanese Patent Application Laid-Open No. 55-86103(1980), ferromagnetic non-acicular particles having a large magnetization, an appropriate coercive force and an appropriate average particle size have recently been demanded as a magnetic recording material, in particular, as a perpendicular magnetic recording material.
Generally, Ba ferrite particles are known as ferromagnetic non-acicular particles.
A method of autoclaving an aqueous alkaline suspension containing Ba ions and Fe(III) by using an autoclave as a reaction apparatus (this method is hereinunder referred to as "autoclaving method") is hitherto known as a process for producing plate-like Ba ferrite.
With respect to the magnetic properties of plate-like Ba ferrite particles suitable for magnetic recording, a coercive force of about 300 to 2,000 Oe is generally required, and in order to reduce the coercive force of the plate-like Ba ferrite particles produced in the autoclaving method to an appropriate coercive force, a method of substituting a part of Fe(III) in the ferrite by Co(II), Ti(IV) or the ions of a divalent metal such as Mn, Zn and Ni has been proposed.
The magnetization of plate-like Ba ferrite particles must be as large as possible, as is described in Japanese Patent Application Laid-Open (KOKAI) No. 56-149328 (1981) ". . . the magneto plumbite ferrite which is used as a magnetic recording material is required to have the greatest possible saturation magnetization. . . ". As a method of enhancing the magnetization of plate-like Ba ferrite particles, for example, a method of bring zinc into solid solution on the surfaces of the plate-like Ba ferrite particles is known [Japanese Patent Application Laid-Open (KOKAI) No. 62-176918 (1987)].
The particle size of plate-like Ba ferrite particles suitable for magnetic recording is required to be as fine as possible, in particular, to be not greater than 0.3 .mu.m. This fact is described, for example, in Japanese Patent Application Laid-Open (KOKAI) No. 56-125219 (1981) ". . . perpendicular magnetic recording is effective in relation to horizontal recording in the region in which the recording wavelength is not more than 1 .mu.m. In order to sufficiently record and reproduce in this wavelength region, the crystalline particle diameter of the ferrite is preferably substantially not greater than 0.3 .mu.m. However, if the particle diameter is about 0.01 .mu.m, the desired coercive force tends to be not exhibited. Therefore, a crystalline-particle diameter of 0.01 to 0.3 .mu.m is appropriate".
There is no end to the recent demand for the improvement of the properties of plate-like Ba ferrite particles, and plate-like Ba ferrite particles having a narrow coercive force distribution in addition to an appropriate coercive force, a large magnetization and an appropriate particle size, as described above are strongly required.
The reason for the demand for a narrow coercive force distribution is that plate-like Ba ferrite particles are apt to have a wide distribution of coercive force due to the non-uniformity of the ingredients, thereby deteriorating the erasing characteristic, as described in the Reports of Technical Researches of the Institute of Electronics and Communication Engineers of Japan MR 77-36 (1978) "Co-doped magnetic iron oxide particles have a serious defect in that when they are formed into a tape, the print through and the erasing characteristic are inferior because the coercive force of the particles is apt to change with heat and with time. These defects are considered to be caused by the fact that the Co ions move in the crystals even at room temperature", and in Japanese Patent Application Laid-Open (KOKAI) No. 61-17426 (1986) ". . . It has been found that as the .gamma.-Fe.sub.2 O.sub.3 particles become finer, the coercive force distribution is enlarged and that the coercive force distribution has a tendency of being further enlarged when the particles are coated with cobalt. . . . Even if the cobalt-coated .gamma.-Fe.sub.O.sub.3 particles are made finer for the purpose of high-density recording, a magnetic powder having a bad coercive force distribution and deteriorated erasing characteristic in spite of a predetermined coercive force (Hc). . . ".
Further, plate-like Ba ferrite particles having a large anisotropy field in addition to a narrow coercive force distribution are strongly required.
The reason for the demand for a large anisotropy field is that such a large anisotropy field enables high-density recording without lowering the output even in a high-frequency band.
This fact is described, for example, High-density Memory Technique and Materials (1984), pp. 67 to 68 "FIGS. 2, 3 and 12 shows the relationship between output and wavelength obtained by recording/reproducing with a ring head by using a Co-Cr single-layer medium having a large anisotropic perpendicular magnetic field (Hk). An excellent high-density recording characteristic such as D.sub.50 =135 KBPI is obtained. . . ".
As a method of enhancing the magnetization of plate-like composite Ba ferrite particles, for example, a method of modifying the surfaces of the plate-like composite Ba ferrite particles by spinel ferrite (Japanese Patent Application Laid-Open (KOKAI) Nos. 60-255628 (1985), 60-255629 (1985), 62-139121 (1987), 62-139122 (1987), 62-139123 (1987) and 62-139124 (1987), and U.S. Pat. No. 4,584,242), and a method of solid-solving zinc in the neighbourhood of the surfaces of plate-like composite Ba ferrite particles in the form of a solid solution (Japanese Patent Application Laid-Open (KOKAI) Nos. 62-176918 (1987), 62-265121 (1987) and 63-2812 (1988) are conventionally known.
Plate-like Ba ferrite particles having an appropriate coercive force, a large magnetization, an appropriate, particle size, a narrow coercive force distribution and a large anisotropy field have now been strongly demanded, but plate-like Ba ferrite particle obtained by the above-described, known method of improving the magnetization show a maximum magnetization. In addition, according to the former known method, the anisotropy field is as small as about 2 to 3 KOe and according to the latter known method, the coercive force distribution is disadvantageously wide.
Thus, the establishment of a process for producing plate-like composite ferrite containing Ba particles having a large magnetization and a narrow coercive force distribution is now in strong demand.
As a result of various researches on a process for producing plate-like composite ferrite containing Ba particles having a large magnetization and a narrow coercive force distribution, it has been found that the plate-like composite ferrite containing Ba particles obtained by mixing an aqueous solution of a ferric salt, Ba salt and M salts, wherein M represents at least one metal (II) selected from the group consisting of Co, Ni, Zn, Mn and Mg and at least one metal (IV) selected from the group consisting of Ti, Sn, Zr and Ge, and an aqueous alkaline solution in a temperature range of 50.degree. to 150.degree. C. to produce fine alkaline mixture particles, filtering out the fine alkaline mixture particles, washing them with water, calcining (heat-treating them in a temperature range of 300.degree. to 600.degree. C., suspending the heat-treated fine particles in an aqueous solution containing Fe, Zn and metals M to produce a suspension of less than pH 4, adding an aqueous alkaline solution to the suspension to produce a suspension of not less than pH 4 containing the heat-treated fine particles with a hydroxide of Fe, Zn and the metal M precipitated on the surface thereof, filtering out and drying the obtained fine particles, and calcining (heat-treating) them in a temperature range of 800.degree. to 1,100.degree. C. in the presence of a flux, have a large magnetization and a narrow coercive force distribution. The present invention has been achieved on the basis of this finding.